Vector control is performed by dividing a current of an AC electric motor into a magnetic field (i.e., magnetic flux) direction and a torque direction that is orthogonal to the magnetic field, thereby making it possible to control a magnetic flux current and a torque current, independently, and also to control a developed torque instantaneously as with a DC electric motor.
The vector control requires a velocity sensor in order to measure the magnetic flux. However, an electronic circuit is normally incorporated into the velocity sensor, and an operating temperature range of the electronic circuit is narrower than the operating temperature range of the AC electric motor to which the velocity sensor is attached.
Also, in the case where the electric motor uses both ends of a shaft, or in the case where the installation location of the electric motor is limited, the velocity sensor cannot be fitted to the electric motor. Also, even if the velocity sensor can be fitted to the electric motor, it is preferable to prevent the attachment of the velocity sensor in order to ensure the reliability of the facility since a coupling portion of the velocity sensor with the electric motor is vulnerable to shocks and a low level signal wiring is required.
Under the above circumstances, there has been proposed a vector control apparatus that conducts a velocity estimate on the basis of information other than velocity without using the velocity sensor.
However, at the time of low-speed operation, because the output voltage value is very small, an error occurs in the output angular frequency, and there exists an area where the rotation velocity is unstable.
In order to cope with the above problem, the conventional vector control apparatus for an induction motor includes a magnetic flux generator circuit having a table that sets a magnetic flux command value while referring to a magnetic flux command curve according to a primary angular frequency command in order to reduce the unstable area at the time of the low-speed operation (for example, refer to Patent Document 1).
In this case, a magnetic flux command curve is generated from the magnetic flux generator circuit to prevent an induced voltage torque axis component from becoming a given value or lower even at the time of the low-speed operation. Then, the SN ratio is increased to suppress the induced voltage torque axis component and an arithmetic error in the arithmetic value of the primary angular frequency to smaller values.
However, in conducting the vector control in the above method, when the induction motor that is controlled through the inverter device is changed, it is necessary to change the setting value of the magnetic flux generator circuit in each of the induction motors.
Patent Document 1: JP 08-009697 A